Field of the Invention
The present invention relates to an optical modulator and an operating point control method of an optical modulator.
The present application claims priority on Japanese Patent Application No. 2014-67112, the content of which is incorporated herein by reference.
Description of the Related Art
Optical communication devices operating with wavelengths ranging from 1,310 nm to 1,550 nm have been used for local area networks (LANs) and optical fibers used for household appliances. It is preferable to employ silicon-base optical communication devices in which optical function devices and electronic circuits can be integrated on silicon platforms by way of CMOS technologies.
Silicon-base optical communication devices have been developed and applied to waveguides, optical couplers, wavelength filters, optical modulators, etc. Among them, optical modulators serving as active devices attract attention among engineers. Additionally, it is generally known that Mach-Zehnder interferometers can be applied to optical modulators using changes of refractive indexes. Optical modulators using Mach-Zehnder interferometers are designed to produce optical intensity modulation signals by way of interference using differences of optical phases in arms including two optical waveguides.
Various types of optical devices and optical modulators have been developed and disclosed in various documents. Patent Literature Document 1 discloses an optical phase control circuit which carries out synchronism detection on small modulation components so as to stably control an operating point of an optical modulator. Patent Literature Document 2 discloses a high-speed silicon-base electro-optic modulator. Patent Literature Document 3 discloses an optical waveguide circuit using a Mach-Zehnder interferometer. Patent Literature Documents 4-6 disclose optical modulators. Patent Literature Document 7 discloses a semiconductor laser using optical interference.
FIG. 13 is a schematic illustration showing an example of an optical modulator using a Mach-Zehnder interferometer. The optical modulator includes a first arm 101 and a second arm 102, which are connected to an optical branch structure 103 and an optical coupling structure 104. The optical branch structure 103 is branched into the arms 101 and 102 in the light-input side while the optical coupling structure 104 couples the arms 101 and 102 together in the light-output side. Light input to the optical branch structure 103 is changed in phase while being guided along the arms 101 and 102. Then, optical signals transmitted through the arms 101 and 102 are combined together via the optical coupling structure 104. Both the arms 101 and 102 are silicon-base electro-optic elements which operate based on voltages so that light is changed in phase due to an electro-optic effect or a thermo-optic effect.
Both the arms 101 and 102 have the same length. Without any voltages, no phase differences occur between the arms 101 and 102 so as to superimpose optical signals having the same wavelength, thus maximizing the intensity of light output from the optical coupling structure 104. With a phase difference π occurring between the arms 101 and 102, optical signals transmitted through the arms 101 and 102 are cancelled out when combined together via the optical coupling structure 104, thus minimizing the intensity of light output from the optical coupling structure 104.
Generally speaking, it is possible to maximize an extinction ratio of light by setting an operating point to the intensity of light output from an optical modulator applied with an intermediate voltage between the maximum voltage maximizing the intensity of light and the minimum voltage minimizing the intensity of light. Any one of arms is set to an initial state applied with a voltage causing an optical phase difference corresponding to a half wavelength, and then an operating point (or a reference point) is set to the intensity of light in the initial state. An optical modulator operates based on an operating point so as to output an optical signal. For this reason, it is important to control an operating point constantly. However, it is difficult to stabilize an operating point of an optical modulator due to any changes of environmental temperatures, dispersions of products in manufacturing, and degradation during long-time usage.
Various studies have been carried out to control operating points of optical modulators. Patent Literature Document 1 discloses a technology of controlling an operating point by use of a constant frequency signal superimposed on an operating voltage of a drive circuit causing a phase difference between a first arm and a second arm. Patent Literature Document 5 discloses a technology of controlling an operating point due to a thermo-optic effect using a heater disposed separately from a phase modulator.
For example, operating points may be greatly shifted due to degradation of optical modulators, or operating points may be slightly shifted due to changes of temperatures in optical modulators being driven. The foregoing technologies are unable to control large shifts and small shifts of operating points under low voltages. Additionally, it is difficult to realize optical modulators which can concurrently calibrate large shifts and small shifts of operating points under low voltages.
The technology of Patent Literature Document 1 needs a high voltage above the operating voltage of a drive circuit, causing a phase difference between a first arm and a second arm, since a constant frequency signal is superimposed on the operating voltage of a drive circuit. Since this technology needs to increase the operating voltage of a drive circuit; it is impossible to control an operating point under a low voltage. Additionally, increasing the frequency of a frequency signal superimposed on the operating voltage of a drive circuit may affect the accuracy of phase modulation. In short, this technology is able to solely control a small shift in an operating point of an optical modulator.
The technology of Patent Literature Document 5 utilizing a thermo-optic effect may involve a high phase-change ratio relative to the operating voltage so as to control a large shift in an operating point of an optical modulator. Due to a high phase-change ratio relative to the operating voltage, it is impossible to accurately calibrate a small shift at an operating point of an optical modulator. This may cause a serious problem in mass-produce devices put on the market. It is possible to produce trial products causing high phase-change ratio relative to operating voltages. In trail products, it is possible to calibrate small shifts at operating points by accurately controlling operating voltages. However, it is unpractical to accurately control operating voltages in mass-produce devices put on the market.